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Number 12

In this issue: Localisation of mineralisation

at Coronation Hill, etc.

High-uranium granites and uranium deposits, Kakadu region

Stream-sediment geochemistry, Kakadu Conservation Zone

New results from the Mount Isa Geotraverse

Localisation of the Sons of Gwalia gold deposit, W A

LEONORA (W A) geological sheets released

GDA upgrade Deep-sea polymetallic sulphides Mud Tank Carbonatite Finger-printing diamonds

using nitrogen Workshop on hydrodynamics

of basin fluids BMR Mineral Data Analysis

System (MDA) Northern Drummond Basin

epithermal gold setting Chemical modelling of

Canning Basin Zn-Pb deposits Intraplate Volcanism in Eastern

Australia & New Zealand Do lamprophyres have high

precious-metal contents? Giles Complex, central Australia

Supplement Kalimantan

Geoscientific Data Package Epithermal sinters in

exploration Successful workshop on using fluid

inclusions in exploration

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10 11

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ISSNOgI3 - 751 X © Commonwealth of Australia 1990 April 1990

Localisation of mineralisation in the Coronation Hill and related deposits,

South Alligator Valley Mineral Field, NT Recent studies carried out as part of the BMR Kakadu Conservation Zone project bave led to an improved understanding of the formation of the Au-PGM (platinum-group metal) and uranium deposits of the South Alligator Valley Mineral Field (Needham, 1988: Geology and Mineralisa•tion of the South Alligator Valley Mineral Field, 1 :75 000 Map, BMR) and their regional geologi•cal setting. A distinct combination of host Iitholo· gies, highly oxidised mineralising fluids, and dila•tional structures within a dextral strike-slip fault system all combine to suggest that Coronation Hill (Au-PGM) is an unusual style of deposit. Figure I shows the important geological features that are considered to control the mineralisation.

The deposits lie close to the unconformity between (I) the little-deformed 1860 Ma felsic volcanics and sandstones belonging to the Coro•nation Sandstone of the EI Sherana Group and (2) the underlying highly folded and deformed Kool•pin Formation of the South Alligator Group (for illustrative purposes the folding has been 'subdued ' in Figure I). The uranium mineralisation gener•ally occurs in cherty ferruginous units of the car•bonaceous Koolpin Formation below the uncon•formity , whilst the Au·PGM mineralisation occurs both below and above the unconformity and is hosted by a wider range of rock types. Uranium mineralisation is not known deeper than 100 m

Fig. I. Schematic block diagram showing geolog•ical features controlling mineralisation in the South Alligator Valley Mineral Field.

below the unconformity or more than a few metres above it. On the other hand, the Au-PGM mineral•isation appears to be much more widely distrib•uted both above and below the unconformity.

Detailed structural studies carried out by Dr R.K. Valenta of Monash University at all of the main deposits from Rockhole to Coronation Hill , and also on a regional scale for the whole of the original (2300 km 2) Kakadu Conservation Zone, have shown that the mineralisation is contained within a major northwest-trending dextral strike•slip fault system , most of the deposits being on or near the Rockhole- EI Sherana- Palette fault sys•tem (Fig. 2). Almost all mineralisation is loca•lised around fault zones that are clearly associated with dilatancy, and the deposits are of basically two types. The first type occupies sites related to contractional jogs. In this case, the extension is in a vertical sense, so tensile fracturing and dilatancy is favoured on planes which are close to horizontal and the resultant mineralisation occurs in subho•rizontal pipe-or ribbon-like bodies, e.g. Rockhole, and possible also Saddle Ridge and Sieisbeck (off map, to southeast). In contrast, the other type occurs in features that are formed as a result of dilational jogs. These produce tensile stresses in the horizontal plane perpendicular to the maxi•mum principal stress and consequently the miner•alisation occurs as subvertical pipe-like features, extending to greater depth, e.g. Coronation Hill, Palette, and Skull. Figure 3 illustrates the differen•ces in shape and orientation of the two types of orebodies.

Dilational jog with vertical pipe-like orebody

D EI Sherana Group • Gerowie Tuff 'I "'...-,~, "

tion zones

Koolpin Formation

Mundogie Formation

Quartz veins __ .- Unconformity

Early thrust fault

~ Orebody

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Published by the Australian Government Publishing Service for the

Bureau of Mineral Resources, Geology and Geophysics, Canberra Department of Primary Industries and Energy

BMR Research Newsletter 12

2 km 1

........ Normal fault

............. Reverse fault

~ Strike-slip fault

® u IAu-PGEj deposit

n " m m ;<

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Fig_ 2_ Location of deposits in the major northwest-trending dextral strike-slip fault sys•tem, South Alligator Valley Mineral Field.

All known deposits are surrounded by alteration zones that may extend for over I km away from the mineralisation; the zones have also been found on other faults well away from the known deposits. The a lterat ion is characterised mineralogically by muscovite (serici te)-ch lorite ± kaolinite ± biotite ± hematite. Hematite, which formed late in the a lteration process, is the most extensive type of alteration. At some of the deposits (e.g. EI She•rana) the rocks are strongly desilicified at the unconformity and consist of predominantly ch lorite-rich felsic vo lcanics that still retain some primary volcanic textures such as flow-banding and quartz phenocysts. In other places, generally at higher stratigraphic leve ls above the unconfor•mity, the volcanics are si licified and quartz-veined (e.g. parts of the Coronat ion Hill deposit, Pul Pul Hill). Chemically the alteration is characterised by loss or gain ofSiO" very high Fe3+ / Fe2+ and U/Th ratios, and almost complete depletion of NaJO, CaO, and Th. There is some enri chment of U, but U rarely exceeds 50 ppm, even in the old uranium pits. Studies of the a ltered rocks suggest that they were formed by interaction with relatively low•temperature, oxidised, low-pH, saline fluids.

On geologica l grounds the mineralisation is you nger than 1860 Ma (the age of the EI Sherana Group host rocks) and. as it is contro lled by fau lts that also cut the Kombolgie Sandstone (- 1650 Ma) nearby, it may be younger than 1650 Ma. One of the earliest papers on U-Pb geochro•nology in Austra li a was o n uraninites from the EI Sherana, Palette, and Sie isbeck mines (Green•halgh & Jeffrey, 1959: Geochimica et Cosmochim•ica AC1l1, 16, 39- 57) which yielded ages of between 500 and 900 Ma. New ion microprobe U-Pb data on hydrothermal zircons in a wide var•iety of altered rock types a lo ng the Rockhole - EI Sherana - Palette fault system and elsewhere in the region give similar ages(R.W. Page, pers. comm.).

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Current work is aimed at establishing the signifi- (a) cance of these young zircons relative to the mineralisation.

Structurally, the deposit model described here and shown in Figure I is similar in many respects to that proposed by Johnson & Wall (1984: Abstracts of the Geological Society ofAustrulia, 12, 285 - 287) for the major unconformity-related uranium deposits of the Alligator Rivers Uranium Field to the north (Jabi luka, Ranger, Koongarra, Nabarlek). Although these deposits contain much more U than those of the South Alligator Valley Mineral Field, the Jabiluka and Koongarra depos•its also contain significant Au reserves, although the only PGMs recorded are at Jabiluka, which contains palladium (Wilde & others, in press: Eco•nomic Geology, Monograph 6).

Johnston & Wall (op cit.) suggested that, in this context, the essential role of an unconformity is simply to provide a significant contrast in permea•bi lit y and rheology between the cover and base•ment rocks. Thus. it follows that 'unconformity'•style deposits could form well below the uncon•form ity within the basement sequences at locations where there is a strong rheological and litho log ica l contrast (Fig. I). This concept could be applied to some of the small uranium deposits that occur in the basement sequences well below the projected level of the EI Sherana Group/ Koolpin Formation unconformity on the northwestern extension of the major fault structures northwest of Rockhole (Coirwong Gorge area). None of these prospects have been tested for Au or PGM concentrations.

Despite the sim il arities in structural style between (I) the Coronation Hill deposit and other deposits of the South Alligator Valley Mineral Fie ld and (2) those of the Alligator Rivers Ura•nium Field, there are several important differences:

In the Alligator Rivers Uranium Field the host rocks below the unconformity are the carbo•nate and carbonaceous schists of the lower Cahill Formation or the Myra Falls Metamor•phics and the unit above the unconformity is (or, where missing, is inferred to have been) the Kombolgie Formation (predominantly quartz sandstone).

In the vicinity of the South Alligator deposits there are much lower vol umes of U-enriched rocks (see p.5 , this issue).

Au and PGMs have been found to be we ll above the crusta l average in a greater propor•tion of country rocks in the South Alligator Valley Mineral Field, particularly the black shales and mafic igneous rocks, e.g. Koolpin Formation, Shovel Billabong Andesite, Zamu Dolerite, and Birdie Creek Volcanic Member.

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Fig. 3. The two contrasting orebody styles, South Alligator Valley Mineral Field (end-member spa•tial variations in the geometry of dilatant sites): (a) steep reverse fault with flat dilatant site (e.g. Rockhole, ?Saddle Ridge, ?Sleisbeck); (b) steep strike-slip fault with steep dilatant site (e.g. Coro•nation Hill, Palette area, Skull).

If the geochronological results do confirm that the minera li sation is as young as 600- 900 Ma, then these so lutions would be much younger than the - 1600 Ma age proposed for the Nabarlek , Koongarra, and Jabiluka deposits (Maas, 1989: Economic Geology, 84, 65 - 90) and the so lutions may have parameters that are more favorable to Au and PGM dissolution and tra nsport.

The Coronation Hill type of Au-PGM deposit appears restricted to the South Alligator Valley. Studies carried out by the BMR Proterozoic Framework Project show that Proterozoic strati•graphic sequences, in particular highly folded basement rocks containing carbonaceous and/or ferruginous cherty and/or calcareous units simi lar to those of the Koolpin Formation of the South Alligator Valley, are more common in northwest•ern to central parts of northern Australia (e.g. the Biscay Formation of the Halls Creek Mobile Zone, the Mount Charles beds of the Granites-Tanami Block, and parts of the Warramunga Group of the Tennant Creek Inlier). By contrast, a lteration patt•erns caused by interaction with similar highly oxi•dised, saline, low-pH fluids are more common in northeasterly areas, in the McArthur Basin, Murphy Inlier, and NW Mount Isa Inlier. These two major factors come together in the South Alligator Val•ley and this may explain why, although the Au•PGMs were found at Coronation Hill nearly five years ago (Noranda Pacific Ltd, 1985: Prospectus) no similar deposit has been found in any other Proterozoic province.

For jimher information contact Dr Lesley Wyborn at BMR (Minerals & Land Use Program).

High uranium granites of the Kakadu•NW Arnhem Land region

Are they related to the locally abundant uranium deposits? The Kakadu and northwestern Arnhem Land region (Fig. 4) encompasses two major mineral provinces - the Alligator Rivers Uranium Field and the South Alligator Valley Mineral Field. The region is well known for the major uranium dep•osits of Jabiluka, Ranger, Koongarra , and Nab•arlek which together contain about 15% of the world's low-cost uranium reserves. The region also contains at least 138 other uranium mines, prospects, and occurrences and obviously is anomalously enriched in uranium; it corresponds to a regional uranium high containing some ofthe higher background levels of uranium in Australia.

As part of the Kakadu Conservation Zone Pro•ject, a detai led study has been completed on the poorly known Malone Creek Granite, and the results can now be integrated with the earlier study of Ferguson & others (in Ferguson & Goleby (Edi•tors), 1980: URANI UM tN THE PINE CREEK GEOSYN•CU NE. /AI-A Vienna, 73- 90) on other granites in the region.

Most of the granites in the region have uranium con tents well above the crusta l average of2.8 ppm (based on Taylor & McLennan, 1985: THE CON•TINI' NTAI. CRUST: tTS COMPOSITION AND EVOI.UTtON.